The present invention disclosed herein relates to an active material for a lithium secondary battery composite electrode for improving output and a lithium secondary battery including the active material, and more particularly, to an electrode for a lithium secondary battery and a lithium secondary battery including the electrode, in which, in a composite electrode having two or more active materials mixed therein, an active material having a small particle size forms the composite electrode by being coagulated and secondarily granulated so as to allow mixed active material particles to have a uniform size range, and thus, electrical conductivity is improved to have high output characteristics.
In line with increasing technological development and demand relating to mobile devices, demand for secondary batteries as an energy source has been increased. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle lifetime, and low self-discharge rate have been commercialized and widely used. Also, in line with growing concerns about environmental issues, a significant amount of research into electric vehicles and hybrid electric vehicles able to replace fossil fuel vehicles such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, has been actively conducted. Recently, research into applying lithium secondary batteries having high energy density and discharge voltage as a power source of such electric vehicles and hybrid electric vehicles has been actively conducted and some of the research are in a commercialization stage.
In particular, various types of research into the development of a cathode material of a large capacity lithium secondary battery for an electric vehicle have been conducted in order to replace currently used LiMn2O4, and recently, a great deal of research into using LiNixMnyCo1-x-yO2 ternary layered oxide has been conducted in order to develop a high capacity battery.
However, with respect to the ternary layered oxide, since there are limitations in stability during overcharge, research into a composite electrode using the ternary layered oxide and LiMPO4 (M=Fe, Mn, Co, and Ni) lithium metal phosphate having an olivine structure without O2 discharge during overcharge, in particular, LiFePO4 using Fe, in a cathode active material has been actively conducted in order to resolve such limitations. The foregoing composite electrode has higher capacity in comparison to that of a single component electrode and may be better in terms of lifetime characteristics and overcharge safety. Therefore, a lithium secondary battery used as a power source of medium and large sized devices may be provided.
However, with respect to the composite electrode including LiFePO4 as above, since electrical conductivity may be poor, a technique of preparing an electrode by increasing a content of a conductive agent has been known. However, with respect to the electrode prepared by using the foregoing method, output characteristics of a lithium secondary battery may be difficult to be improved, because large electrical resistance may be manifested during discharge of the lithium secondary battery.
Accordingly, improving electrical conductivity of an electrode has become an important issue in research on a lithium secondary battery. In particular, with respect to a lithium secondary battery used as a power source of medium and large sized devices, since high output characteristics are required and a phenomenon of a rapid decrease in output must be prevented, there is an urgent need for introducing a technique to resolve the foregoing limitations.
Typically, in order to improve conductivity of an electrode formed of a composite having two or more components, a method of adding a larger amount of a conductive agent to the electrode has been attempted.
However, a binder must be added together for bonding between active material particles or disposing the active material on an electrode current collector, in order to form an electrode. The larger the content of the conductive agent included in the electrode increases, the larger the content of the binder having no electrical conductivity also increases. As a result, when large amounts of the conductive agent and the binder are included, a thickness of the electrode may not only increase, but energy density of the electrode may also significantly decrease because an amount of the active material in the electrode is relatively reduced and electrical conductivity may decrease as much as the content of the included binder.
Therefore, typically, a resultant effect of improving electrical conductivity of an electrode active material may be insufficient in spite of adding a large amount of a conductive agent and decreases in capacity and output characteristics of a secondary battery may rather occur.